Flame retardant filler

ABSTRACT

A flame retardant filler having brominated silica particles, for example, imparts flame retardancy to manufactured articles such as printed circuit boards (PCBs), connectors, and other articles of manufacture that employ thermosetting plastics or thermoplastics. In this example, brominated silica particles serve both as a filler for rheology control (viscosity, flow, etc.) and a flame retardant. In an exemplary application, a PCB laminate stack-up includes conductive planes separated from each other by a dielectric material that includes a flame retardant filler comprised of brominated silica particles. In an exemplary method of synthesizing the brominated silica particles, a monomer having a brominated aromatic functional group is reacted with functionalized silica particles (e.g., isocyanate, vinyl, amine, or epoxy functionalized silica particles). Alternatively, a monomer having a brominated aromatic functional group may be reacted with a silane to produce a brominated alkoxysilane monomer, which is then reacted with the surface of silica particles.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates in general to the field of flameretardancy. More particularly, the present invention relates toimparting flame retardancy to manufactured articles such as printedcircuit boards (PCBs), connectors, and other articles of manufacturethat employ thermosetting plastics or thermoplastics.

2. Background Art

In the manufacture of PCBs, connectors, and other articles ofmanufacture that employ thermosetting plastics (also known as“thermosets”) or thermoplastics, incorporation of a filler material aswell as a flame retardant is required for rheology control (viscosity,flow, etc.) and ignition resistance, respectively. Typically, bothattributes are not found in one material. That is, silica particles aregenerally the filler of choice for rheology control, whereas brominatedorganic compounds impart flame retardancy. Consequently, the basematerial (e.g., epoxy resin for PCBs, and liquid crystal polymer (LCP)for connectors) properties are compromised because a relatively largequantity of both a filler and a flame retardant is necessary to achievethe desired properties.

Therefore, a need exists for an improved mechanism for imparting flameretardancy to manufactured articles such as PCBs, connectors, and otherarticles of manufacture that employ thermoplastics or thermosets.

SUMMARY OF THE INVENTION

In accordance with the preferred embodiments of the present invention, aflame retardant filler having brominated silica particles, for example,imparts flame retardancy to manufactured articles such as printedcircuit boards (PCBs), connectors, and other articles of manufacturethat employ thermosetting plastics or thermoplastics. In this example,the brominated silica particles serve both as a filler for rheologycontrol (viscosity, flow, etc.) and a flame retardant. In an exemplaryapplication, a PCB laminate stack-up includes conductive planesseparated from each other by a dielectric material that includes a flameretardant filler comprised of brominated silica particles. In anexemplary method of synthesizing the brominated silica particles, amonomer having a brominated aromatic functional group is reacted withfunctionalized silica particles (the particle surface is functionalizedto contain a functional group such as isocyanate, vinyl, amine, orepoxy). Alternatively, a monomer having a brominated aromatic functionalgroup may be reacted with a silane to produce a brominated alkoxysilanemonomer, which is then reacted with the surface of silica particles.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred exemplary embodiments of the present invention willhereinafter be described in conjunction with the appended drawings,where like designations denote like elements.

FIG. 1 is a process flow diagram illustrating a method for synthesizingfunctionalized silica particles to be brominated in accordance with thepreferred embodiments of the present invention.

FIG. 2 is a catalytic cycle diagram illustrating an olefin metathesis bywhich a vinyl-terminated silica particle can be modified withallyltriphenyl silane in accordance with the preferred embodiments ofthe present invention.

FIG. 3 is a block diagram illustrating an exemplary printed circuitboard (PCB) having layers of dielectric material that incorporate aflame retardant filler in accordance with the preferred embodiments ofthe present invention.

FIG. 4 is a block diagram illustrating an exemplary laminate stack-up ofthe PCB shown in FIG. 3.

FIG. 5 is a block diagram illustrating an exemplary connector having aplastic housing that incorporates a flame retardant filler in accordancewith the preferred embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the preferred embodiments of the present invention, aflame retardant filler having brominated silica particles, for example,imparts flame retardancy to manufactured articles such as printedcircuit boards (PCBs), connectors, and other articles of manufacturethat employ thermosetting plastics or thermoplastics. In this example,the brominated silica particles serve both as a filler for rheologycontrol (viscosity, flow, etc.) and a flame retardant. An exemplaryprinted circuit board (PCB) implementation of the present invention isdescribed below with reference to FIGS. 3 and 4, while an exemplaryconnector implementation is described below with reference to FIG. 5.However, those skilled in the art will appreciate that the presentinvention applies equally to any manufactured article that employsthermosetting plastics (also known as “thermosets”) or thermoplastics.

As described below, the brominated silica particles may be synthesizedby, for example, reacting a monomer having a brominated aromaticfunctional group and functionalized silica particles (e.g., the particlesurface is functionalized to contain a functional group such asisocyanate, vinyl, amine or epoxy). This first pathway to preparebrominated silica particles in accordance with the preferred embodimentof the present invention is exemplified by reaction schemes 1, 3 and 4,below. However, those skilled in the art will appreciate that brominatedsilica particles in accordance with the preferred embodiments of presentinvention may be synthesized using other processes and reaction schemes.For example, a monomer having a brominated aromatic functional group maybe reacted with a silane to produce a brominated alkoxysilane monomer,which is then reacted with the surface of silica particles to synthesizethe brominated silica particles. This second pathway to preparebrominated silica particles in accordance with the preferred embodimentof the present invention is exemplified by reaction scheme 6, below.

Functionalized silica particles from which brominated silica particlesin accordance with the preferred embodiments of the present inventionare produced may be either obtained commercially or synthesized.Isocyanate functionalized silica particles, for example, are eithercommercially available or can be readily prepared by reacting anorganosilane, such as isocyanate-terminated alkoxysilane with a silicaparticle. For example, alkoxysilanes or chlorosilanes can be condensedon the surface of a silica particle to yield a silica particlecontaining numerous pendant functional groups.

Likewise, monomers having a brominated aromatic group suitable forreacting with functionalized silica particles to produce brominatedsilica particles in accordance with the preferred embodiments of thepresent invention may be either obtained commercially or synthesized.

While the preferred embodiments of the present invention are describedbelow in the context of a flame retardant filler having bromated silicaparticles, a flame retardant filler in accordance with the presentinvention may more generally comprise halogenated silica particles(e.g., chlorinated silica particles) as well as silica particles thatincorporate non-halogen (e.g., phosphorous) species that impart flameretardancy. For example, any of a number of organohalogen compounds(OHCs) may be reacted with functionalized silica particles (or,alternatively, the OHC may be reacted with a silane, which then may bereacted with the surface of silica particles) in accordance with thepresent invention. These OHCs may be either obtained commercially orsynthesized.

FIG. 1 is a process flow diagram illustrating a method 100 forsynthesizing functionalized silica particles to be brominated inaccordance with the preferred embodiments of the present invention.Silica particles are easily functionalized via a silane coupling agent.Use of a vinyl-terminated silane coupling agent, for example, enablesthe incorporation of a monomer having a brominated aromatic functionalgroup to impart flame retardancy.

Typically, a coupling agent is used to join two disparate surfaces. Inthe manufacture of printed circuit boards (PCBs), a silane couplingagent is often used to join a varnish coating (e.g., an epoxy-basedresin) to a substrate (e.g., glass cloth) to define a laminate, orlaminated structure. The silane coupling agent typically consists of anorganofunctional group to bind to the varnish coating and a hydrolyzablegroup that binds to the surface of the substrate. In particular, thealkoxy groups on the silicon hydrolyze to silanols, either through theaddition of water or from residual water on the surface of thesubstrate. Subsequently, the silanols react with hydroxyl groups on thesurface of the substrate to form a siloxane bond (Si—O—Si) and eliminatewater.

In accordance with the preferred embodiments of the present invention,silane coupling agents suitable for purposes of the present inventioninclude (without limitation) vinyl, isocyanate or amine-terminatedtrialkoxysilanes. One skilled in the art will appreciate, however, thatthe silane coupling agent is not limited to an isocyanate, vinyl oramine-terminated trialkoxysilane. As illustrated in FIG. 1, the method100 begins with the hydrolysis (step 105) of the silane coupling agent,i.e., RSi(OCH₃)₃, to produce RSi(OH)₃. In accordance with the preferredembodiments of the present invention, R is a functional group (e.g.,vinyl, isocyanate or amine-terminated) reactive with a monomer having abrominated aromatic functional group. Methanol is a byproduct of thehydrolysis step 105. The method 100 continues with condensation (step110) of the RSi(OH)₃ to produce a compound represented by the followingformula.

Water is a byproduct of the condensation step 110. The method 100continues with hydrogen bonding (step 115) of the condensate onto thesurface of a silica particle to produce a silica particle represented bythe following formula.

The method 100 concludes with bond formation (step 120) through theapplication of heat to produce a functionalized silica particlerepresented by the following formula.

Water is a byproduct of the bond formation step 120. Each of these steps(i.e., steps 105-120) is performed using procedures well known to thoseskilled in the art.

In embodiments described below, synthesis of brominated silica particlesin accordance with the present invention is demonstrated through the useof isocyanate functionalized silica particles (e.g., synthesized using aisocyanate-terminated trialkoxysilane), hydrogen functionalized silicaparticles (e.g., synthesized using a hydrogen-terminatedtrialkoxysilane), and vinyl functionalized silica particles (e.g.,synthesized using a vinyl-terminated trialkoxysilane). However, similarreaction schemes may be employed using, for instance, a glycidylepoxy-or cycloaliphatic epoxy-terminated trialkoxysilane. In this case, thependant epoxy group can react directly with the hydroxyl group ofbrominated p-cumylphenol (i.e., an exemplary monomer having a brominatedaromatic functional group, discussed below), for example, via suitablecatalysts.

The functionalized silica particle shown in FIG. 1 is subsequentlyreacted with any of a number of reactants to yield either the brominatedsilica particle, directly, or a precursor that can be brominated viasynthetic procedures well known to those skilled in the art. Any of anumber of monomers having a brominated aromatic functional group may beused. For example, U.S. Pat. No. 5,777,007 to Kagawa et al., entitled“BROMINATED P-CUMYLPHENOL FLAME-RETARDANTS FOR RESIN COMPOSITION”,issued Jul. 7, 1998, which is hereby incorporated herein by reference inits entirety, discloses the preparation of brominated p-cumylphenol andits use as a flame retardant for resin compositions. This particularmonomer (i.e., brominated p-cumylphenol) is represented by the followingformula (where a=1 to 3 and b=1 to 2).

A reaction scheme (reaction scheme 1) follows for synthesizingbrominated silica particles in accordance with the preferred embodimentsof the present invention by reacting isocyanate functionalized silicaparticles and a monomer having a brominated aromatic functional group.Here too, a=1 to 3 and b=1 to 2.

Only a single coupling reaction is illustrated in the above reactionscheme 1 for the sake of clarity. However, it is typically desirable tomaximize the Br content of the brominated silica particles by reactingall of the available isocyanate groups. Generally, stoichiometricquantities of the reactants may be used. However, the relative quantityof the reactants may be adjusted to achieve a desired level of Brcontent of the brominated silica particles. The above reaction scheme 1is performed at room temperature using conventional procedures wellknown to those skilled in the art.

A vinyl-terminated monomer having a brominated aromatic functional groupfor use in accordance with the preferred embodiments of the presentinvention is represented by the following formula (where a=1 to 3 andb=1 to 2).

A reaction scheme (reaction scheme 2) follows for the preparation of theabove vinyl-terminated monomer having a brominated aromatic functionalgroup (formula 5). Here too, a=1 to 3 and b=1 to 2.

Reaction scheme 2 reacts brominated p-cumylphenol (formula 4) andbrominated propylene. Generally, stoichiometric quantities of thereactants may be used. This reaction, which occurs in the presence ofpotassium carbonate (KCO₃) and tetrahydrofuran (THF), is performed atroom temperature using conventional procedures well known to thoseskilled in the art.

A reaction scheme (reaction scheme 3) follows for synthesizingbrominated silica particles in accordance with the preferred embodimentsof the present invention by platinum catalyzed coupling onto hydrogenfunctionalized silica particles using the above vinyl-terminated monomerwith a brominated aromatic functional group (formula 5). Here too, a=1to 3 and b=1 to 2.

Only a single coupling reaction is illustrated in the above reactionscheme 3 for the sake of clarity. However, it is typically desirable tomaximize the Br content of the brominated silica particles by reactingall of the available hydrogen groups. Generally, stoichiometricquantities of the reactants may be used. However, the relative quantityof the reactants may be adjusted to achieve a desired level of Brcontent of the brominated silica particles. The above reaction scheme 3is performed at room temperature using conventional procedures wellknown to those skilled in the art.

A reaction scheme (reaction scheme 4) follows for synthesizingbrominated silica particles in accordance with the preferred embodimentsof the present invention by metal metathesized coupling onto vinylfunctionalized silica particles using the above vinyl-terminated monomerwith a brominated functional group (formula 5).

Only a single coupling reaction is illustrated in the above reactionscheme 3 for the sake of clarity. However, it is typically desirable tomaximize the Br content of the brominated silica particles by reactingall of the available vinyl groups. Generally, stoichiometric quantitiesof the reactants may be used. However, the relative quantity of thereactants may be adjusted to achieve a desired level of Br content ofthe brominated silica particles. This reaction, which occurs in thepresence of Grubbs' second generation (G2) catalyst and dichloromethane(DCM), is performed at room temperature using conventional procedureswell known to those skilled in the art.

Although the present invention has been illustrated via the forgoingsynthetic pathways, it is understood that any of a number of reactionschemes may be invoked to brominate a silica particle. For instance, thevinyl-terminated silica particle can be modified with allyltriphenylsilane via coupling reaction such as olefin metathesis. A suitablecatalytic cycle for olefin metathesis that may be used in this regard isillustrated in FIG. 2.

FIG. 2 is a catalytic cycle diagram illustrating an olefin metathesis200 by which a vinyl-terminated silica particle is modified withallyltriphenyl silane in accordance with the preferred embodiments ofthe present invention. In FIG. 2, R₁ is a vinyl functional triphenylcompound such as triphenylsilane or 1,1-diphenyl-4-pentenylbenzene. R₂is the vinyl functionalized silica particle. M is a metal catalystsuitable for olefin metathesis, such as Grubbs' second generation (G2)catalyst. The product of the coupling reaction is denoted with referencenumeral 205.

The product of the coupling reaction 205 is subsequently brominatedusing n-bromosuccinimide, Br₂ or aqueous KBr₃. Kumar et al.,“Instantaneous, Facile and Selective Synthesis of Tetrabromobishenol Ausing Potassium Tribromide: An Efficient and Renewable BrominatingAgent”, Organic Process Research & Development 2010, Vol. 14, No. 1,2010, pp. 174-179 (Published on Web Dec. 30, 2009), which is herebyincorporated herein by reference in its entirety, discloses techniquesfor brominating conventional flame retardants using aqueous KBr₃.

Another example of a monomer having a brominated aromatic functionalgroup for use in accordance with the preferred embodiments of thepresent invention is a brominated alkoxysilane monomer such as thatrepresented by the following formula.

A reaction scheme (reaction scheme 5) follows for synthesizing the abovebrominated alkoxysilane monomer (formula 6).

A first reaction step of the above reaction scheme 5 is a couplingreaction that utilizes Karstedt's catalyst. Generally, stoichiometricquantities of the coupling reactants may be used. In a second reactionstep of the above reaction scheme 5, the coupled reaction product of thefirst reaction step is brominated using Br₂ in the presence oftetrahydrofuran (THF). To prepare a flame retardant filler, thebrominated alkoxysilane monomer would then be reacted with the surfaceof the silica particle as described for other alkoxysilane monomers.

In another embodiment, tetrabromo bisphenol A (TBBPA) is incorporatedonto the silica particle surface by first preparing the (meth)acryloylderivative, and then reacting that compound with a vinyl-terminatedsilica particle. In a similar manner, p-cumylphenol may be reacted with(meth)acrylylchloride, and then with a vinyl-terminated silica particlewhich would subsequently be brominated via any of the commonly employedmethods known to those skilled in the art.

As mentioned earlier, in lieu of synthesizing brominated silicaparticles by reacting a functionalized silica particles and a monomerhaving a brominated aromatic functional group, the monomer may bereacted with a silane to produce a brominated alkoxysilane monomer,which is then reacted with the surface of silica particles to synthesizethe brominated silica particles. This second pathway to preparebrominated silica particles in accordance with the preferred embodimentof the present invention is exemplified by the following reaction scheme(reaction scheme 6) and may be used in lieu of the first pathway(exemplified in reaction schemes 1, 3 and 4, above).

In the above reaction scheme 6, a=1 to 3 and b=1 to 2. A first reactionstep of the above reaction scheme 6 reacts brominated p-cumylphenol andbrominated propylene to produce a vinyl-terminated monomer having abrominated aromatic functional group. This first reaction step isidentical to reaction scheme 2, above. A second reaction step of theabove reaction scheme 6 produces a brominated alkoxysilane monomer byplatinum catalyzed coupling of the product of the first reaction stepand a silane. Generally, stoichiometric quantities of the couplingreactants may be used. In a third reaction step of the above reactionscheme 6, the brominated alkoxysilane monomer product of the secondreaction step is reacted with the silica particle surface to produce abrominated silica particle. The above reaction scheme 6 is performedusing conventional procedures well known to those skilled in the art.

FIG. 3 is a block diagram illustrating an exemplary printed circuitboard (PCB) 300 having layers of dielectric material that incorporate aflame retardant filler in accordance with the preferred embodiments ofthe present invention. In the embodiment illustrated in FIG. 3, the PCB300 includes one or more module sites 305 and one or more connectorsites 310. FIG. 4 is a block diagram illustrating an exemplary laminatestack-up of the PCB 300 shown in FIG. 3. The configuration of the PCB300 shown in FIG. 3 and its laminate stack-up shown in FIG. 4 are forpurposes of illustration and not limitation.

As illustrated in FIG. 4, the laminate stack-up of the PCB 300 includesconductive planes (e.g., voltage planes 405 and signal planes 410)separated from each other by dielectric material 415. For example, thevoltage planes 405 include power planes P3, P5, P7, . . . , while thesignal planes 410 include signal planes S1, S2, S4, . . . . Inaccordance to the preferred embodiments of the present invention, one ormore of the layers of the dielectric material 415 includes a filler ofbrominated silica particles that imparts flame retardancy.

Each layer of dielectric material (e.g., the dielectric material 415) ofa PCB typically includes a varnish coating (e.g., an FR4 epoxy resin, abismaleimide triazine (BT) resin, or a polyphenyleneoxide/trially-isocyanurate (PPO/TAIC) interpenetrating network) appliedto a glass fiber substrate (e.g., woven glass fiber) having its surfacemodified by a silane coupling agent (e.g., typically consists of anorganofunctional group to bind to the varnish coating and a hydrolyzablegroup that binds to the surface of the glass fiber substrate, such asvinylbenzylaminoethylaminopropyl-trimethoxysilane ordiallylpropylisocyanurate-trimethoxysilane). In accordance with thepreferred embodiments of the present invention, a flame retardant fillercomprised of brominated silica particles, for example, is incorporatedinto the varnish coating to impart flame retardancy.

FIG. 5 is a block diagram illustrating an exemplary connector 500 havinga plastic housing 505 that incorporate a flame retardant filler inaccordance with preferred embodiments of the present invention. In theembodiment illustrated in FIG. 5, the connector 500 in configured tomake electrical contact with the connector site 310 (shown in FIG. 3) ofthe PCB 300. Also in the embodiment illustrated in FIG. 5, the connector500 includes a cable 510. The configuration of the connector 500 shownin FIG. 5 is for purposes of illustration and not limitation.

In accordance with the preferred embodiments of the present invention, aflame retardant filler comprised of brominated silica particles, forexample, is incorporated into the plastic housing 505 to impart flameretardancy. The base material of the plastic housing 505 may be, forexample, liquid crystal polymer (LCP) or any suitable thermoplastic orthermoset to which the filler is added.

One skilled in the art will appreciate that many variations are possiblewithin the scope of the present invention. Thus, while the presentinvention has been particularly shown and described with reference topreferred embodiments thereof, it will be understood by those skilled inthe art that these and other changes in form and details may be madetherein without departing from the spirit and scope of the presentinvention.

What is claimed is:
 1. A method of making flame retardant filler,comprising the steps of: reacting a silane and a monomer having abromated aromatic functional group to produce a brominated alkoxysilanemonomer; reacting the bromated alkoxysilane monomer and the surface of asilica particle to produce a brominated silica particle.
 2. The methodas recited in claim 1, further comprising the step of: reacting bromatedp-cumylphenol and brominated propylene to produce the monomer having abromated aromatic functional group.
 3. A method of making flameretardant filler, comprising the steps of: reacting bromatedp-cumylphenol and brominated propylene to produce a vinyl-terminatedmonomer having a bromated aromatic functional group; reacting a silaneand the vinyl-terminated monomer having a bromated aromatic functionalgroup to produce a brominated alkoxysilane monomer; reacting thebromated alkoxysilane monomer and the surface of a silica particle toproduce a brominated silica particle.